The fragment spin difference scheme for triplet-triplet energy transfer coupling

Abstract

To calculate the electronic couplings in both inter-and intramolecular triplet energy transfer ͑TET͒, we have developed the "fragment spin difference" ͑FSD͒ scheme. The FSD was a generalization from the "fragment charge difference" ͑FCD͒ method of Voityuk et al. ͓J. Chem. Phys. 117, 5607 ͑2002͔͒ for electron transfer ͑ET͒ coupling. In FSD, the spin population difference was used in place of the charge difference in FCD. FSD is derived from the eigenstate energies and populations, and therefore the FSD couplings contain all contributions in the Hamiltonian as well as the potential overlap effect. In the present work, two series of molecules, all-trans-polyene oligomers and polycyclic aromatic hydrocarbons, were tested for intermolecular TET study. The TET coupling results are largely similar to those from the previously developed direct coupling scheme, with FSD being easier and more flexible in use. On the other hand, the Dexter's exchange integral value, a quantity that is often used as an approximate for the TET coupling, varies in a large range as compared to the corresponding TET coupling. To test the FSD for intramolecular TET, we have calculated the TET couplings between zinc͑II͒-porphyrin and free-base porphyrin separated by different numbers of p-phenyleneethynylene bridge units. Our estimated rate constants are consistent with experimentally measured TET rates. The FSD method can be used for both intermolecular and intramolecular TET, regardless of their symmetry. This general applicability is an improvement over most existing methodologies.

Advances in molecular quantum chemistry contained in the Q-Chem 4 program package

“…The solvent polarisation effect on the Coulomb coupling is also implemented in this revision, using the efficient PCM kernel in Q-CHEM (Section 6.1). Such results allow one to analyse the physical contributions to the EET coupling [329,331,332].…”

“…Computational schemes that offer total EET couplings such as the fragment excitation difference and the fragment spin difference has been available in Q-CHEM for several years [329][330][331]. In this revision, we have included the option to compute the Coulomb and exchange couplings to further dissect the total coupling and derive physical insights into its origin [332].…”

A simplified approach for the coupling of excitation energy transfer

“…In the comparison of the couplings for triplet-to-triplet energy transfer, the CIS energy gap method [20] and ''fragment spin difference'' scheme [30] proposed by Hsu are employed. The former takes half of the energy gap between the lowest two adiabatic triplet excited states as the coupling strength, while the latter uses the spin population difference to calculate the coupling regardless of the symmetry.…”

Communication: Adjusting charge transfer state energies for configuration interaction singles: Without any parameterization and with minimal cost

“…2,3 By implication, this means that CIS often does a decent job of predicting relative energies between non-CT excited states. Other attractive features of CIS include: (i) it is variational; (ii) it is computationally cheap; (iii) it recovers the correct −1/r asymptotic behavior of CT states that comes about because of the Coulombic attraction between electron attachments and detachments.…”